Owing to its high electrical conductivity, silver powder is widely utilized as a conductive filler in a variety of silicone rubber compositions including addition reaction curing type silicone rubber compositions, condensation reaction curing type silicone rubber compositions, and peroxide vulcanizing type silicone rubber compositions. Since silicone rubber compositions having silver powder blended therein cure into silicone rubber with a low electrical resistance, they are used in the application where electrical conductivity and heat resistance are required. The silver powder blended in silicone rubber compositions usually takes the form of particles and flakes.
As a general rule, silver powder has a strong tendency to agglomerate. The silver powder which is stored for a long term is unsuitable to add to silicone rubber compositions because it is difficult to disperse the agglomerated silver powder during compounding. An improvement in this regard is desired. Another problem is that the cured silicone rubber has a volume resistivity which is unstable.
In particular) flake silver powder is utilized in forming low-resistance (or high conductivity) silicone rubber. In order to facilitate compounding, silver powder is often treated with a chemical agent during commutation. Such chemical agents are saturated or unsaturated higher fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, and oleic acid, metal soaps, higher aliphatic amines and polyethylene wax. This treatment, however, has a possibility to retard vulcanization of silicone rubber compositions to which treated silver powder has been added.
It has recently been considered to use silver powder-loaded connectors instead of U-shaped metal wire connectors for providing connection between electronic circuit boards. The silver powder-loaded connectors include a plurality of layers of a conductive elastomer and an insulating elastomer which are stacked in a zebra pattern, and provide a stable contact, avoiding a point contact and display failure.
However, silver powder used as the conductive element tends to agglomerate and becomes difficult to added to elastomers after a long-term storage as mentioned above. If agglomerated silver powder is compounded, dispersion becomes poor, resulting in a resistance instability and variation.
When elastomers are stacked in alternating layers to construct a zebra connector, the poorly dispersed silver powder can cause a puncture phenomenon that upon pressing in a block form for vulcanization, tearing occurs within conductive layers or at the interface between a conductive layer and an insulating layer. It is then very difficult to consistently manufacture such connectors on a mass scale.